In certain load-controlled braking apparatus, such as shown and disclosed in published German Patent Application DE-AS No. 27 39 884, which corresponds to British Pat. No. 2,003,566, there is equipment for controlling the brakes of vehicles. In this previous system, the load-controlled braking apparatus controls a braking pressure specified for the wheel brakes of one or more vehicle axles in relation to a control pressure originating from a brake valve by means of a control valve in accordance with a ratio which is a function of the load status of the vehicle.
The regulation is accomplished by a control piston which controls the control valve having a control surface pressurized by the control pressure and a reaction surface on which the regulated pressure is exerted. The size of the reaction surface can be varied as a function of the load. The control ratio is a function of the surface ratio between the reaction surface and the control surface which varies with the size of the reaction surface which is also a function of the load.
In the same housing as the control valve, there is provided a relay valve which can be activated by means of a relay piston which is pressurized directly in the open direction of the relay valve by the pressure released by the control valve. The relay valve releases from a reservoir via brake connections so that braking pressure on the wheel brakes is essentially identical to the pressure released by the control valve.
Thus, a family of characteristic curves of this load-controlled brake apparatus consists of a family of straight lines with each corresponding to a load status of the vehicle.
There are applications for vehicles in which, on account of government requirements, for example, there is a need in emergency situations for the brake pressure released in the vehicle to be gradually controllable, up to the full available delivery pressure. Since on vehicles with load-controlled braking systems of the type described above, the brake pressure cannot increase above that corresponding to the current load status, this requirement can only be met on vehicles equipped in this manner with additional separate valve and/or switching equipment.